Nanoplastics Stimulate Colony Formation and Bloom in Phaeocystis globosa

The genus Phaeocystis is a globally distributed harmful alga that adversely impacts marine ecosystems. Nanoplastics (NPs) are ubiquitous in marine environments and often co-occur with Phaeocystis blooms. However, the effect of NPs on the colony formation and bloom dynamics of Phaeocystis remains poorly understood. Here, we conducted both indoor and outdoor mesocosm experiments to demonstrate that NPs significantly increased the total cell density, colonial diameter, and colonial density of P. globosa. These enhancements in colonial density and diameter were primarily driven by elevated levels of extracellular polysaccharides as well as key substrates involved in polysaccharide synthesis, including glucose 1-phosphate, adenosine diphosphate glucose, and uridine diphosphate glucose. Concurrently, the accumulation of extracellular polysaccharides was accompanied by the significant upregulation of ten genes associated with N-glycan synthesis. Furthermore, assuming no other environmental changes, our projections indicated that if 0.1% of microplastics (MPs) degrade into NPs, colonial cell densities of Phaeocystis could increase by more than 5% at 242 global locations, with potential maxima approaching 80%. These findings indicate that the ongoing rise in MPs pollution could exacerbate Phaeocystis blooms by enhancing colony formation, posing substantial risks to marine ecosystems.